Millennia ago, eight great mystic entities entered into the Wager of the Octessence to determine who among them was the most powerful. Each of them created a totem which held a fraction of their vast power, and would transform the first human who touched into an exemplar, a living embodiment of that power. The entities all then had Temples created to house their totems. Once a mortal became an exemplar, a spell would be triggered, drawing others to the temples, creating eight Exemplars in all.
Iron Man Vol. 3 #21
Thor Vol. 2 17
Iron Man Vol. 3 #22
Peter Parker: Spider-Man Vol. 2 #11
Juggernaut Vol. 2 #1 (1999)